Explosive growth of the optical communication traffic in recent years, driven by bandwidth hungry applications and progress in transmission technologies alike, has greatly contributed to ever-increasing demand for higher capacity optical networks offering more flexibility at lower cost. Two different yet complimentary trends are becoming more and more evident in this respect. First, deep penetration of the optical fiber into the access networks and, second, greater demand for capacity, bandwidth provisioning and agility back up into the upper layer networks. Both require massive deployment of the optical gear that drives the traffic along the fiber links, on a scale not seen in earlier generation networks. Specifically, optical transceivers, which receive downstream and send upstream data signals, have to be deployed at every optical line terminal or/and network user interface in the access optical networks, but they also are the key optical components to be installed at every node of opaque (local or metropolitan area) optical networks. Whereas performance requirements, e.g. in terms of speed, optical power or sensitivity, for such optical components may be relaxed as compared to their upper layer networks counterparts, cost efficiency and volume scalability in manufacturing are increasingly becoming the major requirements for their mass production
Photonic integrated circuits (PICs), in which multiple elements of common or different functionalities are monolithically integrated onto one chip, are an attractive solution to mass production of highly functional optical components in that they enable scalable volume manufacturing by means of semiconductor wafer fabrication techniques. The PICs offer the ability to dramatically reduce the component footprint, avoid multiple packaging issues, eliminate multiple optical alignments and, eventually, create economical conditions in which optical components achieve the cost efficiency and volume scalability enabling to transfer them into consumer photonics products. The advantages of PIC technology become especially compelling when active waveguide devices, such as laser or photodetector, are combined with the passive waveguide circuitry to form a highly functional photonic system on the chip. Since the active devices usually all are made from artificially grown semiconductors having bandgap structure adjusted to the function and wavelength range of their particular application, such semiconductors are the natural choice for the base material of the PICs. For example, indium phosphide (InP) and related III-V semiconductors are the common material system for the PICs used in optical fiber communications, since they uniquely allow the active and passive devices operating in the most important wavelength windows around 1555 nm and 1310 nm to be combined onto the same InP substrate.
In the context of applications in the optical networks, PIC should be designed for operation in specific wavelength range(s), which are pre-determined, broadly, by physical properties of silica fibers (e.g. minimum loss in 1555 nm range and minimum dispersion in 1310 nm range), and, narrowly, by wavelength plan of a particular optical link (e.g. operating wavelengths sit on ITU wavelength grid within the fiber-limited ranges). With ever-increasing demand for higher capacity and flexibility of the optical networks, on one hand, and growing role of wavelength division multiplexing (WDM) and bandwidth provisioning as means to respond to those demands, on the other hand, the trend in wavelength requirements is clearly towards very narrow and well-controlled operating wavelength windows. The PIC design must be compliant with these requirements and yet should remain suitable for cost-efficient manufacturing with low complexity and high yield.
The well-known solution to wavelength-selective design of active (e.g. lasers and modulators) and passive (e.g. wavelength filters and elements of optical input/output) integrated photonics components is the use of periodic optical structures operating on the Bragg reflection principle and commonly referred to as waveguide Bragg gratings (WBGs). Physically, a waveguide Bragg grating is a one-dimensional periodic perturbation of propagation medium, which creates conditions for a wavelength-selective bi-directional coupling between waves propagating along the grating axis. From their functional properties point of view, Bragg gratings are almost ideally suited to applications in guided optics, which is that all the planar PICs are based upon, since they allow for wavelength-selective filtering, distributed Bragg reflection (DBR) and distributed feedback (DFB) in a course of waveguide propagation. Disregarding to the waveguide and WBG designs, periodic perturbation of the optical waveguide in the direction of propagation results in a bidirectional coupling in the wavelengthsλm=(2neff/m)Λ, m=1,2,3  (1)where neff is the effective index of the optical waveguide mode and Λ is the period of the grating. The coupling efficiency, however, does depend on the waveguide and WBG designs and, more specifically, is directly proportional to the overlap between the waveguide mode and the WBG. Therefore, from the WBG performance prospective, it is always advantageous to maximize this overlap by positioning the grating where the maximum optical field is positioned. On the other hand, this may be not necessarily the optimum solution from the fabrication complexity and yield points of view. A compromise between the performance and manufacturability should be based on a practical application need, such that if the PIC is a cost saving solution, the WBG has to perform adequately and yet be fabricated in a cost-efficient manner.
There are different techniques for processing the Bragg gratings in a context of a planar technology, but by far the most popular one is that, in which corrugated grating is defined by optical beam interference on a flat interface between the layers forming the waveguide core. For example, this technique is commonly used in a fabrication of high-performance DFB semiconductor lasers, since it enables the best overlap between the optical mode and the grating to be achieved to the advantage of the device threshold current, speed and other characteristics. However, it requires re-growth to complete the device fabrication and is very demanding as it concerns to the quality of the interface between the etched corrugated surface and newly grown epitaxial material, which injection current should pass through with minimal loss. Therefore, this technique is not a particularly low-complexity or high-yield solution.
An alternative approach, developed in the previous art precisely with a purpose to eliminate costly and low-yield re-growth processes form fabrication of the grating-based waveguide devices, DFB lasers in the first place, is related to so-called laterally coupled (LC) optical gratings. In this technique, historically proposed for, but not limited to the semiconductor ridge DFB lasers, the WBG is typically defined by holographic or electron beam or optical lithography on, or beside, or instead of the ridge waveguide sidewalls. In this way, the device fabrication can be completed by using only one growth step, which indeed is a significant and very attractive advantage from the manufacturing yield and cost points of view, but the overlap between the waveguide mode and the optical grating is not particularly strong since the latter is positioned at periphery of the former and the mode interacts with the grating only though its evanescent field.
The most elegant and fabrication-friendly solution for LC grating design known from the previous art and hereafter referred to as the effective-ridge laterally coupled surface etched grating (LC-SEG), is that in which the lateral optical confinement of the ridge waveguide is provided by and combined with the optical Bragg grating formed by two sets of narrow trenches etched from the top surface of the ridge, along the propagation direction and at a certain distance from one to the other.
An early example of such a design is described in the paper by L. M. Miller et al, “A Distributed Feedback Ridge Waveguide Quantum Well Heterostructure Laser”, Technology Lett., Vol. 3, No 1, PP. 6-8, 1991. The authors, by using direct e-beam lithography and reactive ion etching (RIE), were able to fabricate third- and fifth-order DFB lasers in InGaAs—GaAs—AlGaAs material system, operating in ˜1050 nm wavelength, and be the first to demonstrate the effective-ridge LC-SEG design in work.
Later on, similar effective-ridge LC-SEG technique has been extended towards the GaInAsP—InP (e.g. H. Abe et al, “1.55 mm Surface Grating Strained MQW-DFB Laser”, Ext. Abstr., 58th Annual Meet. Jpn. Soc. Applied Physics, P. 1111, 1997) and AlGaInAs—InP (e.g. J. Wang et al, “1.55-μm AlGaInAs—InP Laterally Coupled Distributed Feedback Laser”, IEEE Photon. Technology Lett., No 7, PP. 1372-1374, 2005) material systems. Longer wavelength second- and third-order DFB lasers operating in 1550 nm wavelength range have been fabricated by using holographic lithography and RIE or/and inductively-coupled plasma (ICP) etching, without re-growth steps.
Another example of the previous art, based on a similar design idea, is given in the paper by B. Reid et al, “Narrow Linewidth and High Power Distributed Feedback Lasers Fabricated without a Regrowth Step”, Proc. of European Conference on Optical Communications 2003 (ECOC 2003), Rimini, 2003. In this paper, the third-order DFB lasers operating in 1470 nm-1490 nm wavelength range were fabricated in GaInAsP—InP material system, by using commercial optical stepper lithography and ICP etch process. In this particular previous art, lateral dimension of the grating was limited to only a couple of micrometers (exact number not reported by the authors), such that in fact the device combined the effective-ridge and corrugated ridge designs.
The last usually is used to term a different version of LC grating design, in which the lateral optical confinement is provided by a conventional ridge waveguide structure, where, however, the side walls (vertical or tilted) of the ridge are processed to form a corrugated surface (e.g. Y. Watanabe et al, “Laterally Coupled Strained MQW Ridge Waveguide Distributed-Feedback Laser Diode Fabricated by Wet-Dry Hybrid Etching Process”, IEEE Photon. Technology Lett., Vol. 10, No 12, PP. 1688-1690, 1998; Watanabe, U.S. Pat. No. 6,714,571 B2). The LC corrugated gratings are more complex to fabricate and, after all, not directly related to the present invention, which is limited to the LC-SEG designs.
From the teachings of the previous art, it is clear that the effective-ridge LC-SEG indeed is a highly manufacturable technique, implementable in one growth step, by using different means of lithography and methods of etching, proven on a wide range of materials. On the flip side, because of the very nature of the LC-SEG, which interacts with the optical waveguide mode only through its evanescent field, the devices based on this grating design suffer from the lack of coupling efficiency that limits their practical utilization. For example, in the case of DFB lasers, the weaker coupling results in a higher radiative loss or/and longer cavity length, which, eventually, increases the threshold current or/and reduced the modulation speed. It would be advantageous therefore to provide a solution removing the constraints of the prior art, by offering the design that enhances the coupling efficiency of the LC-SEG while preserving the low-complexity and high-yield fabrication advantages associated with this design concept.